<!DOCTYPE html>
<style>
  div {
    border: 2px solid black;
    padding: 5px 0; /* No horizontal padding as outline-offset is not affected by it and span simulates it */
    font-size: 16px;
  }

  span {
    display: block;

    /* 3/6px offset for border */
    height: 2px;
    width: calc(100% - 20em + 6px);
    margin: -1px calc(10em - 3px); /* -1px vertical to remove height of span from div height */
    background: red;
  }
</style>

<p>PASS if there is a thin red line in the middle of the box.</p>
<div>
  <span></span>
</div>